Surface planarization of thin silicon films during and after processing by the sequential lateral solidification method

ABSTRACT

Systems and methods for reducing a surface roughness of a polycrystalline or single crystal thin film produced by the sequential lateral solidification process are disclosed. In one arrangement, the system includes an excimer laser ( 110 ) for generating a plurality of excimer laser pulses of a predetermined fluence, an energy density modulator ( 120 ) for controllably modulating the fluence of the excimer laser pulses such that the fluence is below that which is required to completely melt the thin film, a beam homoginizer ( 144 ) for homoginizing modulated laser pulses in a predetermined plane, a sample stage ( 170 ) for receiving homoginized laser pulses to effect melting of portions of the polycrystalline or single crystal thin film corresponding to the laser pulses, translating means for controllably translating a relative position of the sample stage ( 170 ) with respect to the laser pulses, and a computer ( 110 ) for coordinating the excimer pulse generation and fluence modulation with the relative positions of the sample stage ( 170 ) to thereby process the polycrystalline or single crystal thin film by sequential translation of the sample stage ( 170 ) relative to the laser pulses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/979,201, filed Feb. 4, 2002, now U.S. Pat. No. 6,830,993, issued Dec.14, 2004, which is a national stage of International ApplicationPCT/US00/07479, filed Mar. 21, 2000, each of from which priority isclaimed.

NOTICE OF GOVERNMENT RIGHTS

The U.S. Government has certain rights in this invention pursuant to theterms of the Defense Advanced Research Project Agency award numberN66001-98-1-8913.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to techniques for semiconductorprocessing, and more particularly to semiconductor processing which maybe performed at low temperatures.

II. Description of the Related Art

In the field of semiconductor processing, there have been severalattempts to use lasers to convert thin amorphous silicon films intopolycrystalline films. An overview of conventional excimer laserannealing technology is presented by James Im et al. in “Crystalline SiFilms for Integrated Active-Matrix Liquid-Crystal Displays,” 11 MRSBulletin 39 (1996). In systems used for carrying out excimer laserannealing, an excimer laser beam is shaped into a long beam which istypically up to 30 cm long and 500 micrometers or greater in width. Theshaped beam is scanned over a sample of amorphous silicon to facilitatemelting thereof and the formation of polycrystalline silicon uponresolidification of the sample.

The use of conventional excimer laser annealing technology to generatepolycrystalline or single crystal silicon is problematic for severalreasons. First, the silicon generated in the process is typically smallgrained, of a random microstructure, and/or has non-uniform grain sizes,which result in poor and non-uniform devices that lead to lowmanufacturing yield. Second, the processing techniques needed to obtainacceptable performance levels require that the manufacturing throughputfor producing polycrystalline silicon be kept low. Also, these processesgenerally require a controlled atmosphere and preheating of theamorphous silicon sample, which lead to a further reduction inthroughput rates. Finally, the fabricated films generally exhibit anunacceptable degree of surface roughness that can be problematic forperformance of microelectronic devices.

There exists a need in the field to generate higher qualitypolycrystalline silicon and single crystal silicon at greater throughputrates. As well, there exists a need for manufacturing techniques thatreduce the surface roughness of such polycrystalline and single crystalsilicon thin films to be used in the fabrication of higher qualitydevices, such as flat panel displays.

SUMMARY OF THE INVENTION

An object of the present invention is to provide techniques forplanarizing the surfaces of polycrystalline and single crystal thin filmsemiconductors.

A further object of the present invention is to provide surfaceplanarization techniques that may be applied as a post processing stepto polycrystalline and single crystal thin film semiconductors that areproduced during a sequential lateral solidification process.

Yet a further object of the present invention is to provide surfaceplanarization techniques that may be applied as a processing step duringthe production of polycrystalline and single crystal thin filmsemiconductors in a sequential lateral solidification process.

Yet another object of the present invention is to provide techniques forthe fabrication of high quality semiconductors devices useful forfabricating displays and other products.

In order to achieve these objectives as well as others that will becomeapparent with reference to the following specification, the presentinvention provides systems and methods for reducing surface roughness ofa polycrystalline or single crystal thin film that had previously beenproduced by the sequential lateral solidification process. In onearrangement, the system includes an excimer laser for generating aplurality of excimer laser pulses of a predetermined fluence, an energydensity modulator for controllably modulating the fluence of the excimerlaser pulses such that the fluence is below that which is required tocompletely melt the thin film, a beam homogenizer for homogenizingmodulated laser pulses in a predetermined plane, a sample stage forreceiving homogenized laser pulses to effect partial melting of portionsof the polycrystalline or single crystal thin film corresponding to thelaser pulses, translating means for controllably translating a relativeposition of the sample stage with respect to the laser pulses, and acomputer for coordinating the excimer pulse generation and fluencemodulation with the relative positions of the sample stage to therebyprocess the polycrystalline or single crystal thin film by sequentialtranslation of the sample stage relative to the laser pulses. Theexcimer laser is preferably an ultraviolet excimer laser for generatingultraviolet excimer laser pulses.

In one arrangement, the beam homogenizer is operable to shape laserpulses with a tophat profile in both the x and y directions. The energydensity modulator is operable to attenuate fluence of the excimer laserpulses to approximately 25% to 75% of the full melt threshold of thepolycrystalline or single crystal thin film.

The translating stage advantageously includes an X direction translationportion and a Y direction translation portion, each being coupled to thecomputer and to each other and permitting movement in two orthogonaldirections that are perpendicular to a path formed by the laser pulses,and being controllable by the computer for controllably translating thesample in both of said translatable directions under control of saidcomputer. Also, the beam homogenizer is operable to shape said laserpulses with a tophat profile in both the x and y directions, and thetranslating means is operable to translate the polycrystalline or singlecrystal thin film in two directions orthogonal to a direction of saidlaser pulses such that sequential homogenized laser pulses are incidenton slightly overlapping regions of the polycrystalline or single crystalthin film in the two directions.

In an alternative arrangement, the present invention provides forsystems and methods for processing an amorphous silicon thin film sampleinto a single or polycrystalline silicon thin film having a reducedsurface roughness. In one arrangement, the method includes forming arigid cap layer on an amorphous silicon thin film sample havingsufficient thickness to withstand contractions and expansions duringmelting and resolidification of the silicon thin film during thesequential lateral solidification process. The method also includesgenerating a sequence of excimer laser pulses; controllably modulatingeach excimer laser pulse in the sequence to a predetermined fluence;homogenizing each modulated laser pulse in the sequence in apredetermined plane; masking portions of each homogenized fluencecontrolled laser pulse in the sequence to generate a sequence of fluencecontrolled pulses of patterned beamlets, irradiating the amorphoussilicon thin film sample with the sequence of fluence controlledpatterned beamlets to effect melting of portions thereof; controllablysequentially translating the sample relative to each of said fluencecontrolled pulse of patterned beamlets to thereby process the amorphoussilicon thin film sample into a single or polycrystalline silicon thinfilm having a reduced surface roughness; and removing said cap layerfrom the processed single or polycrystalline silicon thin film.

The accompanying drawings, which are incorporated and constitute part ofthis disclosure, illustrate a preferred embodiment of the invention andserve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional diagram of a system for performing the sequentiallateral solidification process preferred to implement a preferredprocess of the present invention;

FIG. 2 is a chart showing the surface profile of a typical film whichhas been processed by the sequential lateral solidification system ofFIG. 1;

FIG. 3 is a functional diagram of a preferred system for planarizing thesurface of a polycrystalline or single crystal thin film semiconductorproduced during a sequential lateral solidification process inaccordance with the present invention;

FIGS. 4 a and 4 b are illustrative diagrams of a crystallized siliconfilm to be processed by the system of FIG. 3 using a narrow beam;

FIG. 5 is an illustrative diagram of a crystallized silicon film to beprocessed by the system of FIG. 3 using a wide beam;

FIGS. 6–7 are charts showing the surface profile of a typical filmbefore and after processing by the system of FIG. 3;

FIG. 8 is an illustrative diagram of a cross section of a crystallizedsilicon film processed by the system of FIG. 1 in accordance with asecond embodiment of the present invention;

FIG. 9 is a chart showing the surface profile of a typical film whichhas been processed in accordance with the second embodiment of thepresent invention.

FIG. 10 is a flow diagram illustrating the steps implemented in thesystem of FIG. 3 in accordance with the first embodiment of the presentinvention; and

FIG. 11 is a flow diagram illustrating steps implemented in the systemof FIG. 1 in accordance with the second embodiment of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides techniques for planarizing the surfacesof polycrystalline and single crystal thin film semiconductors. In thepreferred embodiments, the surface planarization techniques are appliedas a post processing step to polycrystalline and single crystal thinfilm semiconductors that are produced during a sequential lateralsolidification process, or as a processing step during the production ofpolycrystalline and single crystal thin film semiconductors in asequential lateral solidification process. Accordingly, in order tofully understand those techniques, the sequential lateral solidificationprocess must first be appreciated.

The sequential lateral solidification process is a technique forproducing large grained silicon structures through small-scaleunidirectional translation of a silicon sample in between sequentialpulses emitted by an excimer laser. As each pulse is absorbed by thesample, a small area of the sample is caused to melt completely andresolidify laterally into a crystal region produced by the precedingpulses of a pulse set.

A particularly advantageous sequential lateral solidification processand an apparatus to carry out that process are disclosed in ourco-pending patent application Ser. No. 09/390,537, filed Sep. 3, 1999,entitled “Systems and Methods using Sequential Lateral Solidificationfor Producing Single or Polycrystalline Silicon Thin Films at LowTemperatures,” the disclosure of which is incorporated by referenceherein. While the foregoing disclosure is made with reference to theparticular techniques described in our co-pending patent application, itshould be understood that other sequential lateral solidificationtechniques could readily be adapted for use in the present invention.

With reference to FIG. 1, our co-pending patent application describes asa preferred embodiment a system including excimer laser 110, energydensity modulator 120 to rapidly change the energy density of laser beam111, beam attenuation and shutter 130, optics 140, 141, 142 and 143,beam homogenizer 144, lens system 145, 146, 148, masking system 150,lens system 161, 162, 163, incident laser pulse 164, thin silicon filmsample 170, sample translation stage 180, granite block 190, supportsystem 191, 192, 193, 194, 195, 196, and managing computer 100 X and Ydirection translation of the silicon sample 170 may be effected byeither movement of a mask 710 within masking system 150 or by movementof the sample translation stage 180 under the direction of computer 100.

As described in further detail in our co-pending application, anamorphous silicon thin film sample is processed into a single orpolycrystalline silicon thin film by generating a plurality of excimerlaser pulses of a predetermined fluence, controllably modulating thefluence of the excimer laser pulses, homogenizing the modulated laserpulses in a predetermined plane, masking portions of the homogenizedmodulated laser pulses into patterned beamlets, irradiating an amorphoussilicon thin film sample with the patterned beamlets to effect meltingof portions thereof corresponding to the beamlets, and controllablytranslating the sample with respect to the patterned beamlets and withrespect to the controlled modulation to thereby process the amorphoussilicon thin film sample into a single or polycrystalline silicon thinfilm by sequential translation of the sample relative to the patternedbeamlets and irradiation of the sample by patterned beamlets of varyingfluence at corresponding sequential locations thereon.

While the sequential lateral solidification process is highlyadvantageous to produce single crystal or large grained polycrystallinesilicon thin films, the produced crystals often exhibit a surfaceroughness due to the irrative nature of the melting and resolidificationinherent in the crystal growth process. Thus, as shown in FIG. 2, a 200nm thick crystal will exhibit variations in height throughout the lengthof the crystal. In FIG. 2, a height of 0 indicates the optimal height ina 200 nm thick crystal, and heights varying from 175 to 225 nm are shownto be common throughout the length of the crystal. Note the large bump210 near the crystal boundary, where crystal thickness exceeds theoptimal 200 nm thickness by 350 nm.

Referring to FIGS. 3 and 4, a first embodiment of the present inventionwill now be described. FIG. 3 illustrates a post processing systemembodiment for planarizing polycrystalline and single crystal thin filmsemiconductors produced by the sequential lateral solidificationprocess. The system includes an excimer laser 310, beam attenuator andshutter 320, reflecting plate 330, telescoping lenses 331, 332,reflecting plate 333, beam homogenizer 340, condensing lens 345,reflecting plate 347, field lense 350, sample 360, sample translationstage 370, optical table 380, and managing computer 300. A preferredlaser 310, attenuator 320, telescoping lenses 332, 332, homogenizer 340,and sample translation stage 370 that is movable in two orthogonaldirections are each described in the co-pending patent application Ser.No. 09/390,537. The table 380 may be as described in that patentdocument, or may be an ordinary table. It is preferable that thehomogenized beam 346 be shaped with a tophat profile in both the x and ydirections, and essential that the beam energy density is below thatrequired to completely melt the sample 360.

With reference to FIGS. 4 a and 4 b, the sample 360 is shown in greaterdetail. Since the sample in this embodiment has already been processed,it already includes a large number of single crystal regions, shownillustratively as chevron shaped crystals 365. The homogenized beam 346is shown incident upon a portion 361 of sample 360 to induce partialmelting thereof.

For a 200 nm thick silicon thin film, the full melt threshold isapproximately 600 mJ/cm2. Thus, to induce sufficient partial melting ofthe portion 361, a beam 346 having an energy that is approximately 25%to 75% of the full melt threshold should be utilized. If the beam ismore energetic, energy fluctuations inherent in excimer lasers createthe possibility of causing a full melt of the sample region 361. If thebeam is less energetic, the sample portion 361 will not meltsufficiently to satisfactorily planarize.

As shown in FIG. 4 b, the sample 360 includes a silicon oxide base layer400 and a silicon layer 410. In accordance with the present invention,the outer surface of silicon layer 410 is caused to melt to a depth 420.Upon resolidification, the rough surface 430 is reformed in a moreplanarized manner.

While a single homogenized beam pulse having an energy that isapproximately 25% to 75% of the full melt threshold is sufficient toinduce partial melting of the region 361, it is preferred that multiplebeam pulses are caused to irradiate every such region. Each subsequentbeam pulse will induce partial melting of the region 361, which uponresolidification will exhibit a more planarized surface. Thus, the useoften beam pulses per region 361 will produce a far smoother surface 430than would the use of a single pulse.

Returning to FIG. 4 a, the sample stage 370 is translated, under thecontrol of computer 300, from right to left to cause the homogenizedbeam 346 to scan the sample 360 from left to right 450 on the top ofsample 360. The stage 370 is then moved in an orthogonal direction(shown as the Y direction) to realign the sample at a new position 460,and translation in the opposite direction is began 470. This processesis repeated until the entire surface of sample 360 has been scanned bythe homogenized beam 346.

When the sample stage is translated in the Y direction, it may beadvantageous to align the homogenized beam to slightly overlap apreviously scanned region of the sample 360. Thus, if the region 361 is1.2×1.2 cm, Y direction translation of 1.15 cm may be utilized to avoidedge effects caused by irregularities in the homogenized beam. Likewise,it is advantageous to cause a slight overlap with X-directiontranslation is being effected.

While the foregoing has been described with respect to a tophat profilesquare homogenized beam, beams of other shapes may be utilized. Thus, asshown in FIG. 5, a wide homogenized beam 500 which is sufficiently wideto eliminated the need for X direction translation may be utilized, withthe benefit of necessitating less movement by the translation stage 360,and adoringly, greater throughput. Likewise, a beam that is shaped witha Gaussian profile in the X direction could be utilized if greateroverlaps between X translations are performed.

As shown in FIG. 6–7, the results of the process described withreference to FIGS. 3–4 a are illustrated. The profile of a sample 360fabricated in accordance with the sequential lateral solidificationprocess is shown in FIG. 6 a. The sample exhibits surface irregularitiesof +/−25 nm from the optimal 200 nm height. As shown in FIG. 6 b, afterpost processing with a single laser pulse in accordance with the presentinvention, those surface irregularities are markedly reduced. Theseresults are alternatively illustrated in FIG. 7, where it is shown >100%decrease in surface roughness caused by post processing in accordancewith the invention herein.

Referring next to FIG. 8, a second embodiment of the present inventionwill now be described. In this embodiment, the surface of silicon thinfilm is kept planarized through the employment of a rigid cap layerduring the sequential lateral solidification process. Thus, FIG. 8 showsa thin silicon sample formed of an approximately 50–200 nm thickamorphous silicon layer 810 deposited on a silicon oxide base layer 820.The sample is capped with a thick second silicon oxide layer 820,approximately 2 microns thick, which is substantially rigid. The caplayer must be sufficiently thick to withstand the contractions andexpansions during melting and resolidification of the silicon layerduring the sequential lateral solidification process.

The sample with cap layer 830 are then used in place of sample 170 inthe lateral solidification process, a complete description of which iscontained in the above mentioned patent application Ser. No. 09/390,537.After such processing, the cap layer 830 is removed from the sample bytraditional wet or dry etching techniques. As shown in FIG. 9, theresults of the process described with reference to FIG. 8 isillustrated.

Referring to FIG. 10, the steps executed by computer 300 to control boththe sequential lateral solidification process of FIG. 1 and the surfaceplanarization process implemented with respect to FIG. 3 will bedescribed. The various electronics of the system are initialized 1000 bythe computer 300 to initiate the process. A sample is then loaded ontothe sample translation stage 1005. It should be noted that such loadingmay be either manual or robotically implemented under the control ofcomputer 300. Next, the sample is processed in accordance with thesequential lateral solidification process using the apparatus of FIG. 11010. The processed sample is positioned for planarization 1015. Thevarious optical components of the system are focused 1020 if necessary.The laser is then stabilized 1025 to a desired energy level andreputation rate, as needed to partially melt the sample in accordancewith the teachings of the present invention. If necessary, theattenuation of the laser pulses is finely adjusted 1030.

Next, translation of the sample is commenced 1035 at a predeterminedspeed and in a predetermined direction, in accordance with thepreviously sequential lateral solidification processed regions of thesample. The shutter is opened 1040 to expose the sample to irradiationand accordingly, to commence the planarization process.

Sample translation and irradiation continues until planarization hasbeen competed 1045, 105, at which time the computer closes the shutterand stops translation 1055, 1060. If other areas on the sample have beendesignated for planarization, the sample is repositioned 1065, 1066 andthe process is repeated on the new area. If no further areas have beendesignated for planarization, the laser is shut off 1070, the hardwareis shut down 1075, and the process is completed 1080.

Referring next to FIG. 11, the steps executed by computer 100 to controlthe crystal growth process with the surface planarization stepsimplemented with respect to FIG. 1 will be described. FIG. 10 is a flowdiagram illustrating the basic steps implemented in the system of FIG. 1using a capped sample as illustrated in FIG. 8. An oxide layer isdeposited on a base 1100. A silicon layer is then deposited on the oxidebuffer layer 1110, and a cap oxide is deposited at the top layer of thesample 1120.

Next, the sample is processed in accordance with the sequential lateralsolidification process using the apparatus of FIG. 1 1030. Afterprocessing, the cap oxide is removed, e.g., by a dilute hydrofluoricacid solution.

The foregoing merely illustrates the principles of the invention.Various modifications and alterations to the described embodiments willbe apparent to those skilled in the art in view of the teachings herein.For example, while removal of the cap layer had been disclosed withrespect to use of a dilute hydrofluoric acid solution, the cap layer maybe removed by any. conventional technique such as dry etching. It willthus be appreciated that those skilled in the art will be able to devisenumerous systems and methods which, although not explicitly shown ordescribed herein, embody the principles of the invention and are thuswithin the spirit and scope of the invention.

1. A method for processing an amorphous silicon thin film sample into asingle or polycrystalline silicon thin film having a reduced surfaceroughness, comprising the steps of: (a) forming a rigid cap layer onsaid amorphous silicon thin film sample having sufficient thickness towithstand contractions and expansions during melting andresolidification of said silicon thin film; (b) generating a sequence ofexcimer laser pulses; (c) controllably modulating each excimer laserpulse in said sequence to a predetermined fluence; (d) homogenizing eachmodulated laser pulse in said sequence in a predetermined plane; (e)masking portions of each homogenized fluence controlled laser pulse insaid sequence to generate a sequence of fluence controlled pulses ofpatterned beamlets; (f) irradiating said amorphous silicon thin filmsample with said sequence of fluence controlled patterned beamlets toeffect melting of portions thereof corresponding to each fluencecontrolled patterned beamlet pulse in said sequence of pulses ofpatterned beamlets; (g) controllably sequentially translating saidsample relative to each of said fluence controlled pulse of patternedbeamlets to thereby process said amorphous silicon thin film sample intoa single or polycrystalline silicon thin film; and (h) removing said caplayer from said single or polycrystalline silicon thin film.
 2. Themethod of claim 1, wherein said excimer laser pulses compriseultraviolet excimer laser pulses.
 3. The method of claim 1, wherein saidstep of forming a rigid cap layer on said amorphous silicon thin filmsample comprises forming a silicon oxide layer on said amorphous siliconthin film sample.
 4. The method of claim 1, wherein said step of forminga rigid cap layer on said amorphous silicon thin film sample comprisesforming a silicon oxide layer approximately 2 microns thick on saidamorphous silicon thin film sample.